Silica Group of Silicate Minerals
|Chemical Composition||SiO2 – Silicon Dioxide|
|Color||Usually clear or white, but minor impurities can create a variety of colors. Crystals are most commonly hexagonal prisms, whose pyramid-like ends often line the faces of open voids. Cryptocrystalline varieties are most often massive.|
|Cleavage||Does not exhibit cleavage, although crystal faces may be mistaken for cleavage planes. Conchoidal fracture is characteristic of both macrocrystalline and cryptocrystalline quartz varieties.|
|Hardness||7 (very hard)|
|Specific Gravity||2.6 to 2.65 (macrocrystalline varieties)
|Luster||Crystals are vitreous (glass-like), massive form is dull or waxy.|
At first glance, clear quartz crystals may appear to be similar to a number of other translucent crystalline minerals, but they can usually be distinguished by their hardness and lack of cleavage.
Calcite can occur in translucent masses that mimic the appearance of quartz, but the two minerals are easily distinguished by their hardness and the fact that calcite reacts easily with dilute acids. Quartz is much harder than calcite and, unlike calcite, cannot be scratched by most metal implements. Quartz also lacks cleavage, while calcite exhibits excellent cleavage in three directions.
Halite has distinct cubic cleavage, and although it may resemble the clear rock crystal variety of quartz, the two can be distinguished by halite�s distinctive taste, cleavage and softness. Quartz cannot be scratched by most metal implements and does not display any cleavage. When struck, quartz breaks like glass, producing sharp edges and shards. Consequently when examining a specimen, you should only look for broken edges rather than try to create your own!
Flourite and amethyst quartz are sometimes confused simply because they both form purple translucent crystals, however the two are easily distinguished by their hardness (fluorite has a hardness of 4 and is easily scratched by a nail, while quartz at 7 is harder than a nail), shape (fluorite crystals are cubic, while quartz are hexagonal) and cleavage. Fluorite exhibits perfect cleavage in four directions to form octahedrons, while quartz lacks any cleavage and breaks by conchoidal fracture.
A few varieties of garnet may be confused with rose quartz, and the two minerals have about the same hardness. Quartz, however, does not exhibit cleavage and if a garnet crystal is well formed it will have a distinctive dodecahedral (twelve-sided) shape.
Did you know...
Quartz was the mineral upon which the Stone Ages were based. With few exceptions, most early stone tools were fashioned of quartz. Outcrops of quartz that were suitable for tool manufacturing were targeted by some of the earliest known mining activities and the mined quartz was traded across vast distances, even before humans began to establish agricultural societies.
Even in our modern world, quartz is one of the most widely used
minerals, though few people are aware of its many contributions. One of
its most common uses is also its most transparent, as quartz is the
source of most of our society’s glass- from windowpanes and crystal
goblets to eyeglasses and cathedrals’ stained glass windows. Quartz is
one of the most common minerals because it is chemically and physically
stable at Earth’s surface conditions. It is a significant component of
hydrothermal veins and felsic igneous rocks, and is often the dominant
mineral in sandstones and siltstones, as well as their metamorphosed
Quartz occurs in a wide range of varieties under a bewildering array of informal names, such as jasper, flint, tiger’s eye, amethyst, citrine, chalcedony, onyx, opal and agate. Because of their abundance, the quartz varieties are often subdivided into informal groups. One of the most common subdivisions is ‘chert’, a term collectively used for all the quartz varieties that have crystals too small to be seen without a microscope. In this use, ‘chert’ is simply a subset of ‘quartz’.
Description and Identifying Characteristics
Despite its variable appearance, Quartz’ hardness (greater than that of glass or most metals) and lack of cleavage make it relatively easy to distinguish from other minerals. Ancient Greeks called quartz ‘krystallos’, which means ‘ice’, a reflection of the clarity of quartz’ clear transparent crystals. Although most quartz crystals are clear and colorless, slight chemical impurities create a number of color varieties, some of which are common enough to have their own names. Translucent purple crystals are called amethyst which has been designated as February’s birthstone. Translucent pink to reddish quartz is known as rose quartz, while translucent gray and cloudy white crystals are respectively called smoky quartz and milky quartz. In a similar manner, quartz that is composed of microscopic crystals (chert) can occur in a wide variety of colors. The most common chert color varieties are flint (dark gray to black), jasper (red to red-brown), and onyx (mixed white and black). Agate is another variety of chert that exhibits distinct bands of color that formed as the chemistry of the fluids moving through its host rock changed slightly.
In Our Earth: The Geologic Importance of Quartz
Quartz is a common component of silica-rich igneous rocks, forming up to 25% of the volume of granites. It also occurs in hydrothermal veins and pegmatites. Because quartz is relatively stable at surface temperatures and pressures, it can be concentrated by weathering processes to be even more abundant in sedimentary rocks than it is in igneous rocks. Quartz sand grains are a significant component of most sandstones and siltstones, and some detrital sedimentary rocks are almost exclusively composed of quartz grains. Metamorphism of these sedimentary rocks produces quartzite, a metamorphic rock composed almost entirely of quartz.
Quartz’ stability, even under harsh chemical conditions, was aptly demonstrated by some dinosaurs that swallowed stones to aid in the digestion of land plants. These gizzard stones, called gastroliths, are almost always composed solely of quartz. Other stones the dinosaur may have swallowed broke down in the swirl of its strong stomach acids, only leaving quartz rocks to survive as working gastroliths.
Although it is chemically resistant, quartz is weakly soluble so most natural waters contain some dissolved silica that can precipitate as quartz. Hence quartz is a common vein-filling and cavity-filling mineral in rocks through which silica-rich waters have moved. Some microscopic marine plankton, like diatoms and radiolarians, take advantage of the dissolved silica in seawater to create shells of quartz. These can accumulate on the sea floor, particularly in deep sea areas. Under very high pressure, water can dissolve calcite and aragonite shells rather easily, while less soluble quartz shells are left to form quartz-rich deposits. Microscopic quartz crystals often form chert seams and nodules as silica-rich groundwater moves through a variety of sedimentary and metamorphic rocks. Chert precipitated from silica-rich groundwater can also replace bone and wood present in sediment and sedimentary rocks, often even preserving the materials’ original cellular structure. This process is the origin of ‘petrified’ wood.
Since quartz occurs in so many different geological settings, it can be associated with many different minerals. In igneous rocks and pegmatites, quartz usually occurs with potassium feldspars, muscovite, biotite and amphiboles. In sedimentary rocks or metamorphosed sedimentary rocks, quartz may be associated with potassium feldspar, calcite, dolomite or a variety of clay minerals.
In Our Society: The Economic Importance of Quartz
So many varieties of quartz have played such crucial roles in human society that untangling the web of names and terms is daunting. The word ‘quartz’ appears to have originated from the German or Saxon languages, but the meaning of the term is unknown.
In our earliest human experience, the microcrystalline varieties of quartz played an unparalleled role in setting our ancestors on the path towards modern civilization. The many varieties of chert were some of the first materials beyond wood and bone to be sculpted into tools. When a piece of chert is struck, it can break with conchoidal fracture to create a sharp edge. Early humans learned to work chert, breaking it systematically to create tools. Now millions of years old, some of these tools still have remarkably sharp edges, a testimony to quartz’ stability. Later, humans learned to work obsidian (volcanic glass) as well, but even through the Neolithic Age, most stone tools were made from chert.
Its use in our modern world is nearly as prevalent. Although glass is one of its most familiar products, our society has a remarkable range of uses for quartz. Large amounts are used as flux in metallurgy and as an abrasive and filler in refractories. Crucibles designed for use at high temperatures are made of fused silica, and pure quartz sands are also used in the manufacture of glass and glass ceramics. Quartz-rich rocks like quartzite and quartz sandstone are often used as building stones and some colored quartz varieties are used as ornamental and semi-precious stones. By volume though, the bulk of all commercially mined quartz is used in the construction industry as aggregate for concrete and as sand in mortar and cement. As a result, from buildings to roads a remarkable amount of our modern infrastructure is built of quartz.
Because of its physical strength, ground quartz is used as an abrasive in stonecutting, sandblasting, and scouring soaps. Since it is chemically stable, crushed quartzite fragments are often used as ballast (finely broken rock fragments) along railroad lines and highway shoulders. Pure, fine quartz sands are also used in water purification systems as a filter since it will not react with the water and the pores between the sand grains are small enough to filter out many impurities. Many species of marine plankton, called diatoms, construct their shells of quartz. As these microscopic organisms die, they sink down to cover the deep sea floor with layers of porous diatom shells. Ancient deep sea diatom deposits now exposed at the Earth’s surface are called diatomaceous earth. Diatomaceous earth has some unusual uses. It is used to filter some of the best wines and as the abrasive part of toothpaste. Since it is composed of quartz, the shells are harder than the apatite and calcite minerals that make up our teeth. When you polish your teeth, you actually are grinding the surface down with a very fine abrasive, often the shells of long dead plankton.
One of the more interesting properties of some pure quartz crystals is that they are piezoelectric – that means that when it is put under pressure, the crystal produces an electric voltage. This characteristic allows quartz crystals to be used to measure pressures or control the frequency of electric impulses, which led to their use in radio systems and timepieces.
Quartz veins are also important host rocks for gold and other precious metals, and were the target of many mining ‘rushes’. From a historic perspective, however, the single most important use of quartz may have been to start fires. When chert is struck against iron, it produces a relatively long-lived spark. From prehistoric times on, this has been one of the most reliable and widespread means of starting a fire until the advent of matches, and was the basic firing mechanism of colonial age flintlock muskets.
Since quartz is hard and comes in many color varieties, the chert forms of quartz are often used a gemstones. Jasper, tiger’s eye, amethyst, and citrine are all varieties of chert used as gems for jewelry. Onyx is a name used for both a banded variety of quartz and a banded variety of calcite, so don’t be confused by the term’s double use. Both types of onyx are used as decorative stone, but differ in their uses. Calcite onyx is much softer than quartz onyx and is more easily carved, so it is used for most ‘onyx’ carvings. Since it is harder, quartz onyx is more frequently used for jewelry and other objects that must withstand greater handling and wear.
Opal is a well-known gemstone often mistaken as a variety of finely crystalline quartz (chert). Like quartz, it is composed largely of SiO2, but with the addition of water [SiO2-(H20)n]. Technically opal is not a true mineral though, but something called a mineraloid. Mineraloids are amorphous solids, which means that they do not have a set crystal structure. To be a true mineral, a solid must have a crystalline structure. Opal is found as a vein-filling or void-filling material in rocks through which silica-rich waters have moved. There are other minerals, besides quartz, that are also composed solely of silicon dioxide (SiO2), but most of these are only stable at high pressure or temperature, so they rarely occur at the Earth’s surface. Although opal is technically neither a variety of quartz nor even a true mineral, in the jewelry trade it is often considered as both.
Although quartz is chemically stable and is non-toxic, like any fine particle quartz dust can be hazardous if it is inhaled. Construction workers using sandblasting equipment or even home repair enthusiasts working with cement dust should use good quality masks to avoid breathing airborne quartz dust.
Quartz in the Upper Midwest:
Quartz is a very common mineral that can be collected in half the counties in Minnesota and Wisconsin. The best-known occurrences of macroscopic quartz are the Paleozoic sandstone units that mantle much of the region. One of the purest quartz sands in the world is the St. Peter Sandstone that covers parts of southeastern Minnesota and southern Wisconsin. Among its many uses, St. Peter Sandstone outcrops in the Twin Cities area were once the source for most of the automobile glass used in Ford Motor Company cars. Lake Superior agate is probably the most famous regional occurrence of chert (quartz composed of microscopic crystals). These agate masses, originated, as silica was precipitated from fluids to fill pore spaces in ancient volcanic lava flows along the northern border between Minnesota and Wisconsin. As the lava rock weathered, the more chemically resistant quartz fillings were freed to roll around on the gravel beaches of Lake Superior as agate.